In various industries, such as the foundary industry and the production of cast iron and steel, it is customary to charge a furnace with scrap metal. In foundaries the scrap metal is almost entirely the source of raw material employed to obtain the casting metal. In the basic iron and steel industry, great quantities of scrap are employed in the furnaces in combination with iron ore.
The type of scrap material used may take various forms. One source of scrap is such items as sheet shearings, punchings, turnings, chips, gates, risers and scraped castings. Other sources of scrap, which have become of increasing use, are scrap automotive parts such as shattered engine blocks, transmissions and differentials.
One of the problems in using scrap is the presence of various contaminants such as oil and grease, especially in the automotive scrap. Where the scrap has been stored outdoors, there will be entrapped moisture present in the scrap. It is undesirable to charge such contaminated scrap into a furnace since the entrapped moisture and other readily vaporizable material, such as grease, will expand rapidly in the furnace with the effect of an explosion. In addition, grease and oil which does not vaporize immediately, can result in contamination of the molten metal.
Charging of cold scrap into a furnace will increase the time for the batch to reach pouring temperature as well as the load on the furnace as compared to preheated scrap. Thus, many foundaries desire preheating scrap to improve furnace output and efficiency.
For the foregoing reasons, various types of equipment have been developed for the purpose of preheating scrap and removing moisture and grease or oil from the scrap. One such type of equipment is a preheater which employs an elongate conveyor which passes scrap under a hood. The hood includes a plurality of burners that direct flames upon the scrap metal on the conveyor to burn off the moisture and grease and preheat the scrap.
One of the problems encountered in metal preheating systems of this type is the pollution in the form of smoke and unburned hydrocarbons which result from operation of the system. One way in which to reduce the amount of unburned hydrocarbons is to permit an excess of air under the hood beyond that needed for the burner flame to aid in complete burning of the grease and oil on the scrap. This could be accomplished by operating the burners with an excess of air or by permitting outside air to enter under the hood. However, the presence of excess air under the hood results in oxidation of metal which is undesirable.
As a result of the oxidation problem, preheating systems of the hood type are generally operated with a reducing atmosphere. The resultant heavy concentration of unburned hydrocarbons are permitted to escape to the atmosphere in some cases. In other cases, they are vented from the furnace and after burners are used to clean up the pollutants in the exhaust gases.
Another problem encountered in the present conveyor-hood type preheating systems is the uneven preheating of the scrap. The thickness of the bed of scrap can vary along the conveyor. Additionally, the spacing of the burners, by necessity, must be spread out along the hood. As a result, the flames from the burners will impinge more directly upon the scrap directly beneath the burners than on the scrap between adjacent burners. Further yet, the flames from the burners only strike the top surfaces of the scrap. The sides and underneath portions of the scrap as well as that positioned deeply in the bed are not engaged as directly by the flames from the burners resulting in less uniform heating and contaminants thereon being removed to a lesser degree.